UNIVERSITY    OF    CALIFORNIA    PUBLICATIONS 


COLLEGE   OF  AGRICULTURE 
AGRICULTURAL  EXPERIMENT  STATION 


THE  RED  SPIDER  OF  CITRUS  TREES 


C.  W.  WOODWORTH 


THE    RED    SPIDER 


BULLETIN     NO.    145 


BERKELEY:   THE  UNIVERSITY  PRESS 

November,  1902 


Benjamin  Ide  Wheeler,  Ph.D.,  L.L.D.,  President  of  the  University. 

EXPERIMENT  STATION  STAFF. 

E.  W.  Hilgard,  Ph.D.,  LL.D.,  Director  and  Chemist. 

E.  J.  Wickson,  M.A.,  Horticulturist,  and  Superintendent  of  Central  Station 
Grounds. 

W.  A.  Setchell,  Ph.D.,  Botanist. 

R.  H.  Loughridge,  Ph.D.,  Agricultural  Geologist  and  Soil  Physicist.  (Soils  and 
Alkali.) 

C.  W.  Woodworth,  M.S.,  Entomologist. 

*M.  E.  Jaffa,  M.S.,  Assistant  Chemist.     (Eoods,  Fertilizers.) 

G.  W.  Shaw,  M.A.,  Ph.D.,  Assistant  Chemist.     (Soils,  Beet-Sugar.) 

George  E.  Colby,  M.S.,  Assistant  Chemist.     (Fruits,  Waters,  Insecticides.) 

Leroy  Anderson,  M.S. A.,  Animal  Industries,  San  Luis  Obispo. 

A.  R.  Ward,  B.S.A..  D.V.M.,  Veterinarian,  Bacteriologist. 

E.  H.  Twight,  B.Sc,  Diplome  E.A.M.,  Viticulturist. 

E.  W.  Major,  B.Agr.,  Dairy  Husbandry. 

A.  V.  Stubenrauch,  M.S.,  Assistant  Horticulturist  and  Superintendent  of  Sub- 
stations. 

*J.  Burtt  Davy,  Assistant  Botanist. 

H.  M.  Hall,  M.S.,  Assistant  Botanist. 

C.  A.  Triebel,  Ph.G.,  Student  Assistant  in  Agricultural  Laboratory. 

C.  A.  Colmore,  B.S,,  Clerk  to  the  Director. 


Emil  Kellner,  Foreman  of  Central  Station  Grounds. 

John  Touhy,  Patron,  )    _ 

_.  _  }-  Tulare  Substation,  Tular< 

Julius  Forrer,  Foreman,) 

R.  C.  Rust,  Patron, 


,    Foothill  Substation,  Jackson. 
John  H.  Barber,  Foreman,  ' 

S.  D.  Merk,  Patron,  )n      ,D  a  ,    .   ..        ^        „  ,  , 

T    XT    „  ,TT    ,  ,  >  Coast  Range  Substation,  Paso  Robles. 

J.  H.  Ooley,  Workman  m  charge,) 

S.  N.  Androus,  Patron,)    _      ,.  _  _..       .«,,..  (   Pomona. 

T    ttt    -nr  -^  r  Southern  California  Substation,    -<    _    i     . 

J.  W.  Mills,  Foreman,  j  (  Ontario. 

V.  C.  Richards,  Patron,  )    _  _,_  A. 

m    T     _,  .  }■   Forestry  Station,  Chico. 

T.  L.  Bohlender,  in  charge,] 

Roy  Jones,  Patron,       \  ^  . 

TTT  „  y  Forestry  Station,  Santa  Monica. 

Wm.  Shutt,  Foreman,)  J  ' 


*  Absent  on  leav< 


The  Station  publications  (Reports  and  Bulletins)  will  be  sent  to  any  citizen 
of  the  State  on  application,  so  long  as  available. 


THE  RED  SPIDER  OF  CITRUS  TREES. 


The  investigation  of  the  red-spider  on  the  citrus  trees  of  southern 
California  was  undertaken  in  response  to  a  very  urgent  request  of  the 
Horticultural  Commissioners  of  Los  Angeles  County,  and  has  been 
conducted  in  cooperation  with  them.  A  large  part  of  the  work  was 
under  the  immediate  charge  of  Mr.  W.  H.  Volck,  a  student  in  the 
Entomological  Department,  who  was  appointed  as  inspector  by  the  Los 
Angeles  County  Board  for  this  purpose.  The  present  bulletin  is  only 
a  report  of  progress,  since  the  investigation  is  still  under  way  and  some 
of  the  most  important  facts  remain  yet  to  be  determined.  The  season 
is  nearly  here  for  the  red  spider  to  resume  its  injury,  and  this  is 
published  in  order  to  place  in  the  hands  of  the  orange  growers  the 
information  already  accumulated,  so  that  they  may  make  use  of  it 
during  the  present  season. 

Red  spiders  have  been  injurious  in  southern  California  for  many 
years,  but  little  attention  has  been  given  to  the  matter,  chiefly  on 
account  of  the  extremely  minute  size  of  these  creatures.  During  the  past 
shipping  season  the  injury  to  the  fruit  caused  by  the  improper  use 
of  distillate  sprays,  awakened  more  than  usual  interest  in  the  subject 
of  spotted  fruit,  including  that  produced  by  other  agencies.  The 
theory  was  advanced,  and  generally  believed  by  the  growers,  that  the  red 
spider  was  accountable  for  one  of  the  commoner  forms  of  spotting  very 
prevalent  during  the  past  season.  The  results  of  the  studies  made 
this  summer  on  its  habits  have  thrown  great  doubt,  if  not  indeed  dis- 
proved the  possibility  of  this  mite  producing  anything  like  this  kind 
of  injury.  There  is  no  doubt  that  its  work,  however,  has  resulted  in 
considerable  losses  to  the  growers  of  oranges  and  lemons  that  have  not 
been  clearly  recognized. 

The  injuries  produced  by  the  red  spider  are  chiefly  of  two  sorts, 
both  dependent  upon  a  diminution  of  the  general  health  of  the  tree. 
The  one  most  readily  recognized  is  the  unusual  or  excessive  dropping 
of  the  fruit  from  the  trees  badly  affected.  Usually  this  dropping  is  not 
to  be  attributed  to  the  work  of  the  mite  alone,  but  other  things 
contribute  to  the  result.  The  other  form  of  injury  still  more  difficult 
to  estimate  is  the  loss  which  results  from  the  decrease  in  the  size  and 
sugar  content  of  the  fruit,  that  cannot  help  but  occur  when  the  leaves 


4  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

are  injured  in  the  way  that  results  from  the  feeding  of  this  mite.  To 
these  may  be  added  the  effects  that  may  occur  in  a  diminution  of  the 
growth  of  the  tree ;  but  it  is  possible  that  the  dropping  of  the  fruit  may 
relieve  the  tree  and  counterbalance,  in  part  at  least,  the  direct  injury. 

The  appearance  of  the  fruit  from  trees  badly  infested  with  the  red 
spider  is  very  characteristic,  and  easily  recognized  when  once  known, 
but  has  not,  as  far  as  we  can  learn,  been  taken  into  consideration  in 
the  grading  of  fruit.  Should  the  market  come  to  recognize  and  begin 
to  discriminate  against  the  paler  fruit,  upon  which  the  mite  has  been 
at  work,  the  losses  would  become  very  large. 

The  Species  Concerned. — The  red  spider  that  attacks  citrus  trees  in 
southern  California  is  an  entirely  different  creature  from  that,  known 
by  the  same  name,  that  occurs,  all  over  the  State,  upon  deciduous  trees, 
particularly  the  stone  fruits.  The  species  also  differs  from  the  one 
most  prevalent  on  citrus  trees  in  Florida,  and  from  the  greenhouse 
pest  which  in  this  State  often  attacks  ornamental  plants  quite  seriously. 
These  latter  species  belong  to  the  same  genus  as  the  mite  on  our  orange 
trees,  and  are  therefore  quite  closely  allied.  Many  other  mites  do 
injury  in  California  and  some  of  them  attack  citrus  trees.  The  best 
known  of  these  is  the  silver  mite,  so  troublesome  in  San  Diego  County, 
which  belongs  to  an  entirely  different  group  than  do  the  red  spiders,  and 
is  capable  of  producing  a  much  greater  direct  loss  to  the  grower  than 
any  other  mite  which  attacks  these  fruits  in  this  State. 

The  red  spider  with  which  we  have  to  deal  was  first  described  by 
Hubbard  in  his  report  on  the  orange  insects  of  Florida.  Curiously 
enough,  Hubbard  supposed  it  to  be  carnivorous,  but  Banks,  in  Bulletin  8 
of  the  technical  Series  of  the  Division  of  Entomology  U.  S.  Depart- 
ment of  Agriculture,  in  monographing  the  members  of  this  group, 
states  that  Hubbard  must  have  been  mistaken,  since  all  the  other 
members  of  the  genus  are  vegetable  feeders.  Our  species,  Tetranychus 
mytilaspidis,  differs  in  a  number  of  respects  from  most  of  the  other 
members  of  the  genus,  particularly  in  the  presence  of  conspicuous 
tubercules  upon  which  the  large  spines  are  borne.  The  only  other 
species  possessing  these  tubercules  and  known  to  occur  in  the  United 
States,  has  not  as  yet  been  reported  from  this  State. 

Life  History. 

The  Egg. — The  eggs  of  the  red  spider  are  very  peculiar  in  the 
possession  of  a  long  slender  stalk  projecting  from  the  middle  of  the 
top  side,  extending  upward  nearly  twice  as  high  again  as  the  egg  itself. 
Not  including  the  stalk,  the  egg  is  perhaps  a  third  wider  than  high,  and 
circular  in  outline  as  seen  from  above.     The  stalk  is  transparent  and 


RED    SPIDER — LIFE    HISTORY.  0 

surmounted  on  the  top  by  a  very  slightly  enlarged  transverse  section, 
to  which  is  attached,  immediately  after  laying,  a  series  of  rather 
regularly  arranged  delicate  silken  threads,  about  fourteen  in  number, 
radiating  from  this  point  to  the  surface  of  the  leaf,  forming  a  series  of 
guys  by  which  the  egg  is  held  firmly  in  position  in  spite  of  the  fact 
that  otherwise  it  is  very  loosely  attached  to  the  leaf.  These  are  shown 
in  Fig.  1.  The  egg- sucking  Coniopterijx  described  in  the  latter  part  of 
this  bulletin,  very  commonly  pushes  the  egg  loose  from  its  attachment 
at  the  base,  in  its  efforts  to  puncture  it,  but  does  not  succeed  in  pushing 
it  off  from  the  plant  on  account  of  the  strength  of  the  guys. 

The  egg  when  first  laid  is  uniformly  bright  red  in  color.  Very 
soon  one  may  observe  a  very  curious  series  of  changes  taking  place  in 
the  substance  of  the  egg,  showing  the  progress  of  the  development  of 
the  young  creature  within.  The  length  of  time  necessary  to  develop 
and  hatch  varies  more  or  less  with  the  temperature,  being  distinctly 
more  rapid  in  summer  than  in  winter,  but  also  varies  a  great  deal  when 
all  the  conditions  are  the  same.  Of  two  eggs  laid  the  same  day  by  the 
same  parent,  and  lying  adjacent  to  each  other  on  the  same  leaf,  one  will 
sometimes  require  nearly  twice  the  time  as  the  other  before  the  young 
mite  appears.  In  general,  the  time  may  be  given  for  the  majority  of 
the  eggs  as  between  one  and  two  weeks. 

The  egg  shell  breaks  open  to  allow  the  exit  of  the  young  mite  in  a 
very  regular  manner.  This  break  occurs  all  the  way  around  the  egg 
at  the  equatorial  line.  The  top  half  is  held  in  position  by  the  guys 
previously  described,  so  that  when  the  shell  is  empty  the  two  halves 
may  retain  almost  their  same  relative  position. 


Fig.  1. 
the  egg  of  the  red  spider  showing  the  stalk  and  silken  guys. 


6  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

The  Young  Mite. — Upon  hatching,  the  creature  which  appears 
resembles  the  adult  mite  very  closely  except  in  size  and  in  the  absence 
of  the  hind  pair  of  legs,  making  the  number  six,  thus  resembling 
insects  to  this  extent.  Many  other  mites  are  six-legged  when  first 
hatched  from  the  egg. 

The  young  red  spider  at  once  begins  to  feed  and  is  very  soon  ready 
to  prepare  for  the  change  of  skin  which  results  in  the  assumption  of 
the  fourth  pair  of  legs.  About  a  day's  feeding  will  furnish  sufficient 
growth  for  this  change.  The  old  skin  is  not  changed  at  once,  but  the 
mite  goes  into  a  resting  condition  which  requires  nearly  as  much  time 
as  does  the  feeding  period.  Mites  when  in  their  resting  condition 
place  themselves  in  a  very  characteristic  attitude.  The  position  of  the 
feet  is  similar  to  that  assumed  when  feeding,  which  is  shown  in  the 
figures  of  the  adult  spider,  but  the  mouth-parts  are  retracted.  The 
color  of  the  whole  anterior  end  of  the  body  is  changed,  becoming  much 
paler  or  entirely  white,  the  red  coloring  being  withdrawn  entirely  from 
this  region.  The  legs  are  pulled  partly  back  into  the  body,  so  that 
one  can  see  the  feet,  which  are  being  remodeled,  lying  within  the  skin 
about  to  be  discarded  and  appearing  as  one  leg  within  another.  At 
this  time,  in  a  specimen  that  has  been  cleared  by  soaking  in  alcohol  and 
especially  if  it  is  also  stained,  one  can  also  very  plainly  see  the  newly 
forming  fourth  pair  of  legs.  These  lie  doubled  up  against  the  ventral 
side  of  the  abdomen  just  behind  the  third  pair. 

After  about  a  day  of  rest,  during  which  the  new  skin  is  formed, 
the  back  of  the  old  skin  splits  and  the  first  eight-legged  stage  appears. 
This  eight-legged  mite  is  scarcely  larger  than  the  newly  hatched  one, 
and  would  not  be  easily  distinguished  but  for  the  number  of  legs  it 
possesses.  The  feeding  and  rest  periods  in  this  stage  cover  three  or 
four  days,  and  there  is  a  very  material  growth  to  be  noted.  When  the 
mite  enters  upon  its  second  rest-period,  preparatory  to  a  second  moult, 
it  has  become  about  half  grown.  The  history  of  the  third  stage  of  the 
mite  is  almost  an  exact  repetition  of  that  of  the  second,  only  that  the 
mite  is  much  larger. 

Feeding  Habits. — The  mouth  parts  of  the  red  spider  consist  of  an 
oval  plate  projecting  forward  above  and  between  the  front  legs,  bearing 
on  the  underside  a  sharp  sword-shaped  vertical  blade,  on  each  side  of 
which  there  is  a  rather  fleshy  palpus  and  a  slender  lancet.  When  the 
mite  has  selected  a  location  and  is  about  to  begin  to  feed,  it  bends  the 
mandibular  plate  down  towards  the  leaf,  pulling  the  whole  organ  at 
the  same  time  into  the  body  and  bringing  the  point  of  the  median 
blade  against  the  surface  of  the  leaf.     The  blade  is  guided  and  held 


RED    SPIDER — LIFE    HISTORY.  7 

in  place  by  the  palpi.  The  mite  now  forces  the  mouth  parts  forward, 
effectively  slitting  the  tissue  of  the  leaf  with  the  blade.  Before 
performing  this  last  operation  it  has  carefully  placed  the  two 
anterior  pairs  of  legs  directly  in  front  of  the  body  in  the  position 
shown  in  the  figures  so  that  they  will  hold  to  the  best  advantage  as 
an  anchorage  while  the  surface  of  the  leaf  is  being  broken.  The  strain 
is  so  hard  that  the  feet  will  often  slip  during  the  process. 

After  the  slit  is  made  the  blade  seems  to  be  withdrawn  and  the 
lancets  inserted,  the  palpi  serving  to  direct  them;  these  lancets  can  be 
thrust  out  and  retracted  by  means  of  muscles  attached  directly  to  their 
bases.  The  juices  of  the  leaf  are  sucked  up  through  a  tube-like  cavity 
between  the  palpi;  and  where  each  slit  is  made  and  the  contents 
pumped  out  a  paler  spot  remains.  Each  spot  is  almost  microscopic 
in  size,  but  the  combined  work  of  many  mites  on  a  leaf  produces  an 
injury  that  is  very  evident.  On  the  leaf  the  color  of  the  spot  is  a  dull, 
pale  green,  which  as  the  leaf  turns  yellowish  becomes  quite  undistin- 
guishable  from  other  parts  of  the  leaf.  On  the  fruit  of  the  orange  the 
appearance  while  it  remains  green  is  the  same  as  on  the  leaf.  As  the 
fruit  ripens  the  spots  do  not  become  invisible  but  are  pale  yellow, 
contrasting  even  more  clearly  than  before  with  the  normal  color  of  the 
fruit.  Spots  made  by  the  mite  on  the  ripe  orange  are  the  same  in  color 
as  those  made  while  the  fruit  was  green. 

The  young  red  spider  feeds  almost  continuously,  except  at  the  rest 
periods  preceding  each  change  of  skin.  It  does  not  stay  at  any  one 
spot  any  great  length  of  time,  but  after  pumping  one  place  dry  will 
walk  a  short  distance  and  then  take  another  meal.  The  time  when 
the  mite  wanders  most  is  just  after  the  last  moult,  which  usually 
occurs  on  an  older  leaf.  The  male  may  remain  on  these  leaves,  but 
the  female  generally  proceeds  to  young  foliage  that  has  grown  during 
the  period  the  mite  was  coming  to  maturity.  In  this  way  the  newer 
foliage  will  seem  to  have  only  adult  females. 

The  Adult  Male. — The  male  is  very  much  smaller  than  the  female, 
though  possessing  legs  of  about  equal  length.  It  is  extremely  active, 
wandering  about,  particularly  on  the  older  leaves  where  the  young 
insects  are  most  abundant,  feeding  from  time  to  time,  but  apparently 
spending  the  major  portion  of  its  life  in  moving.  They  are  not  to  be 
found  at  all,  apparently,  on  the  more  exposed  leaves  of  the  plant,  but 
are  fairly  abundant  on  the  under  sides  of  the  older  leaves.  The  total 
numbers  on  the  tree  are  probably  about  one-half  as  many  as  of  the 
females.  The  male  examines  and  seems  to  work  over  the  resting  stages 
of  the  young  insect  with  its  palpi  very  persistently,  and  will  often 


8  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

remain  for  a  long  time  perfectly  quiescent  beside  one  of  these  young 
individuals.  Indeed  the  most  ready  means  of  recognizing  the  male  is 
by  searching  for  pairs  of  individuals  standing  side  by  side. 


Fig.  2. 
the  male  red  spider. 

The  Adult  Female. — The  appearance  of  the  adult  female  is  well 
shown  in  Figure  3  and  in  the  frontispiece.  It  is  on  this  insect  that 
the  hairs  will  be  seen  to  be  best  developed.  The  exact  arrangement 
of  the  hairs  can  be  made  out  by  the  comparison  of  the  illustration.  The 
hairs  consist  of  two  kinds — short  ones  upon  the  legs  and  a  few  at  the 
extremities  of  the  body,  and  large,  curved  bristle-like  hairs  upon  the 
back.  These  latter  are  densely  beset  with  fine  projections,  which  have 
been  supposed  to  afford  points  for  condensation  of  moisture,  keeping 
the  body  of  the  insect  itself  dry.     The  function  of  the  hairs  appears  to 


RED    SPIDER — LIFE    HISTORY. 


be  largely  that  of  protection  against  the  creature's  enemies.  They  are 
so  placed  that  it  is  very  difficult  for  an  insect  to  reach  the  body  of  the 
mite  with  its  jaws.  The  feeding  habits  of  the  adult  are  the  same  as 
described  for  the  young  insect. 


Fig.  3. 

THE    FEMALE    RED    SPIDER. 

Egg-Lmjing. — The  mite  prefers  a  somewhat  protected  location  for 
egg-laying.  The  place  most  commonly  selected  is  on  the  leaf  surface 
immediately  adjacent  to  the  midrib,  especially  on  the  underside.  Very 
commonly  also,  eggs  will  be  seen  just  beneath  the  edge  of  the  leaf 
where  the  surface  curls  slightly  downward.  Another  favored  place 
for  oviposition  is  the  hollow  on  the  under  side  where  the  membrane 
cups  upward  between  two  veins.     Occasionally  an  egg  will  be  laid  in 


10  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

an  exposed  situation;  eggs  are  found  to  some  extent  upon  the  fruit, 
being  situated  almost  always  in  the  hollow. 

The  number  of  eggs  produced  by  this  insect  depends  to  a  large 
extent  upon  the  length  of  life.  We  have  repeatedly  obtained  as  high 
as  eighteen  eggs  from  a  single  female  in  our  breeding  experiments,  and 
eggs  are  produced  as  rapidly  as  three  per  day  for  considerable  periods. 
It  is  impossible  to  say  whether  the  mite  is  under  the  best  conditions  in 
our  breeding  cages,  and  whether  more  might  not  have  been  produced 
under  perfectly  normal  conditions. 

The  egg  is  laid  while  the  insect  is  actually  feeding,  and  the  process 
has  several  times  been  observed  under  the  microscope.  As  soon  as 
the  egg  is  deposited  upon  the  leaf,  the  insect,  still  keeping  its  mouth 
parts  attached  to  the  leaf,  brings  the  hind  legs  partly  under  the  body, 
lifting  the  latter  high  into  the  air.  At  the  same  time  that  the  body  is 
lifted  the  stem  of  the  egg  is  emitted,  and  the  mite  rests  for  a  moment 
with  the  body  held  in  this  elevated  position.  Then  the  mite  ceases  to 
feed,  works  the  abdomen  about  a  little  at  the  top  of  the  egg  shell, 
apparently  bringing  the  feet  against  the  stem  of  the  eggs  and  moves 
the  body  back  and  forth  to  and  from  the  surface  of  the  leaf.  In  this 
way  the  silken  guys  already  referred  to  are  spun  and  attached.  The 
number  of  guys  placed  around  the  egg  is  considerably  larger  than  the 
number  of  trips  made  by  the  abdomen  from  the  top  of  the  egg  stem  to 
the  surface  of  the  leaf,  so  that  it  is  probable  that  the  legs  aid  in  the 
placing  of  these  latter  upon  the  leaf  surface.  We  were  not  able  to 
satisfy  ourselves  as  to  the  exact  position  of  the  spinerets  from  which 
the  silk  is  produced,  but  have  very  little  doubt,  from  the  process  of 
egg  laying  observed,  that  they  are  situated  near  the  hind  end  of  the 
body.  These  silken  strands  are  emitted  by  the  insect  at  other  times, 
as  well  as  at  egg  laying,  though  not  as  abundantly  as  is  the  case  with 
other  species  of  red  spiders.  They  often  form  quite  a  complete  tent 
over  a  colony  of  young  mites.  The  individual  strands  are  so  delicate 
that  it  is  almost  impossible  to  observe  them.  The  mite  is,  in  all  prob- 
ability, continually  emitting  silk  as  do  true  spiders,  because  when 
suddenly  jarred  from  the  leaf  it  will  in  most  cases  hang  suspended 
in  the  air  by  a  silken  thread,  which  it  uses  to  regain  the  leaf,  in  the 
same  way  that  the  true  spiders  climb  the  strands  of  their  web. 

Natural  Enemies. 

The  experience  of  former  years  in  reference  to  this  mite  is  that  it 
suddenly  increases  in  the  fall  and  decreases  again  in  the  spring,  and 
may,  at  times,  almost  completely  die  out  in  an  orchard  and  not  appear 
again  for  several  years.      This  suggested  at   once  that  the  natural 


RED    SPIDER — NATURAL   ENEMIES.  11 

enemies  of  these  mites  might  be  the  most  important  factor  in  the 
problem.  At  the  beginning  of  the  investigation,  therefore,  a  great 
deal  of  attention  was  given  to  the  subject  of  the  natural  enemies  of 
the  red  spider,  and  a  large  number  of  insects  and  mites  were  found  to 
be  engaged  in  feeding  upon  them.  The  most  important  of  these 
we  will  discuss  below. 

Ladybirds. — The  most  conspicuous  enemies  of  the  red  spider  were 
the  larvae  of  the  ladybirds.  The  largest  of  these  ladybirds  was  the 
common  Hippodamia  convergens,  which  was  observed  in  all  stages  in 
the  orchards  about  Azusa  and  indeed  in  every  part  of  southern  Califor- 
nia visited.  This  insect  lays  its  eggs  in  small  masses  of  about  a  dozen 
eggs  each  almost  anywhere,  commonly  on  the  orange  leaves  or  even  on 
the  fence.  The  eggs  are  elongate,  lying  side  by  side  attached  by  their 
ends.  After  hatching,  the  young  larvae  remain  huddled  over  the  egg 
shells  for  about  a  day,  before  they  scatter  and  begin  to  feed.  After 
scattering,  each  larva  wanders  about  by  itself,  crawling  over  the  leaves 
in  search  of  food.  It  does  not  readily  find  a  red  spider,  but  apparently 
in  an  entirely  accidental  manner  bumps  against  one  while  wandering 
about.  After  finding  a  red  spider,  however,  it  will,  if  hungry,  at 
once  pounce  upon  it  and  devour  the  whole  mite  in  short  order.  If  the 
larva  is  small  or  if  it  is  not  so  hungry,  it  may  merely  eat  out  the 
larger  part  of  the  contents  and  leave  the  rest  of  the  carcass  upon  the 
leaf.  The  young  insects  experience  considerable  difficulty  in  getting 
to  the  body  of  the  mite  between  the  hairs,  but  the  older  individuals 
seem  strong  enough  to  force  the  hairs  out  of  the  way.  These  lady- 
birds appear  to  prefer  the  orange  plant  louse  which  is  also  very 
abundant  upon  the  orange  trees  during  the  winter  season.  After  the 
ladybird  larva  is  fully  fed  it  proceeds  to  pupate  upon  the  leaf,  attach- 
ing itself  by  the  hind  end  of  the  body  in  the  manner  usual  with  the 
ladybirds,  the  pupa  being  formed  within  the  old  larval  skin,  which 
splits  open  as  soon  as  the  pupa  is  complete.  Emergence  of  the  adult 
occurs  in  the  same  way  by  the  long  dorsal  split.  We  have  not 
observed  the  adult  insect  feeding  upon  the  red  spider,  but  have  noted 
it  eating  the  plant  lice. 

Another  ladybird,  a  Rhizobiid,  was  more  or  less  abundant  in 
places,  and  the  larva  was  repeatedly  observed  feeding  upon  the  red 
spider.  It  appears,  indeed,  that  the  red  spider  constituted  the  bulk  of 
the  food  of  this  creature,  both  in  the  larva  and  adult  condition.  These 
lady-birds  were  much  less  numerous  than  the  former  species  and  the 
egg  stage  was  not  observed. 


12  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

Lace-winged  Flies. — The  larvae  of  the  common  lace- winged  fly 
(Chrysopa  calif omica  Coq.?)  were  very  much  more  voracious  than 
the  larvae  of  the  ladybirds.  Had  they  been  as  abundant  as  the  latter 
it  is  possible  they  would  have  had  an  appreciable  effect  upon  the 
numbers  of  the  red  spider,  but  while  they  were  abundant  enough  to 
be  collected  in  numbers  they  could  not  be  considered  as  having  any 
noticeable  effect  upon  the  mite.  The  lace- winged  flies  could  readily  be 
observed  in  all  stages  at  any  time  upon  the  orange  trees.  The  eggs 
are  laid  singly  or  only  a  few  together,  usually  on  the  upper  side  of  the 
leaf.  The  adult  insect  first  exudes  a  long  thread  attached  vertically 
to  the  surface  of  the  leaf,  and  then  deposits  the  egg  upon  the  end  of 
this  stalk.  The  young  insect  that  hatches  can  be  readily  distinguished 
from  all  other  larvae  found  on  the  orange  leaves,  by  the  large  size  of 
the  jaws.  The  body  is  somewhat  spindle-shaped,  the  legs  are  well 
developed  and  the  insect  is  very  active,  devouring  mite  after  mite  in 
rapid  succession.  The  insect  does  not  eat  up  the  mite  it  attacks,  but 
merely  sucks  out  the  contents  of  the  body.  When  full  grown  the 
larva  makes  a  slight  cocoon  and  transforms  into  a  pupa,  from  which 
very  soon  the  adult  lace- winged  fly  emerges. 

The  Coniopteryx. — Rather  closely  related  to  the  lace-winged  fly  is 
an  insect  bearing  the  name  of  Coniopteryx.  The  members  of  this 
group  are  very  rare  insects,  only  a  single  rare  species  being  heretofore 
known  in  the  United  States.  Our  species  was  observed  in  considerable 
numbers  in  the  orchards  about  Azusa  and  appears  to  be  more  important 
than  any  other  of  the  natural  enemies  of  the  red  spider.  It  works  wholly 
upon  the  egg  of  the  mite.  The  coniopteryx  is  a  small,  white-winged 
insect,  suggesting  somewhat  a  large  "white  fly."  The  adult  insect 
moves  about  in  much  the  same  way  as  the  lace-winged  fly,  and  while 
slightly  more  abundant  was  by  no  means  a  conspicuous  object.  One 
point  in  the  life  history  has  not  yet  been  cleared  up, — the  egg  laying 
habits  not  having  been  observed.  The  young  insect  is  a  beautiful 
mottled  black  and  white  larva  with  somewhat  the  same  general  shape 
as  those  of  the  lace- winged  fly,  but  with  a  head  having  protuding  eyes 
and  terminating  in  a  slender  beak  instead  of  large,  spreading,  jaws. 
The  legs  are  very  much  smaller,  though  the  insect  gets  about  quite 
rapidly.  It  uses  the  posterior  end  of  the  body  to  a  large  extent  to 
supplement  the  legs,  there  being  a  sucker-like  organ  at  the  end  of  the 
abdomen.  The  larva  is  continually  at  work  searching  for  the  eggs  of 
the  red  spider,  which  as  soon  as  found  it  proceeds  to  puncture.  The 
egg  shell  is  quite  resistent,  so  it  requires  the  utmost  effort  of  the  insect 
to  accomplish  its  purpose.     As  soon  as  the  shell  is  broken  the  larva 


RED    SPIDER — NATURAL   ENEMIES.  13 

begins  to  suck  out  the  contents,  apparently  projecting  into  it  a 
slender  tongue-like  organ  from  the  end  of  the  proboscis,  by  means  of 
which  it  laps  up  the  last  trace  of  the  coloring  matter  from  the  inside 
of  the  shell,  leaving  it  as  white  as  though  the  mite  had  emerged.  The 
rate  of  feeding  depends  entirely  upon  the  abundance  of  eggs.  When 
placed  upon  an  orange  well  stocked  with  eggs,  and  observed  under  the 
microscope,  one  insect  was  noticed  to  remove  the  contents  from  the  red 
spider  eggs  at  the  rate  of  two  per  minute  for  ten  minutes  at  one  time. 
When  the  Coniopteryx  larva  is  full  grown  it  proceeds  to  spin  a 
double  cocoon  on  the  under  side  of  the  leaf,  consisting  of  an  outer  flat 
layer  about  6  mm.  in  diameter,  under  the  center  of  which  it  makes  a 
nearly  spherical  inner  covering. 

Predaceous  Mites. — The  red  spider  is  attacked  by  three  or  four  kinds 
of  predaceous  mites.  One  large  red  species  was  extremely  active  and 
devoured  the  female  mites  very  voraceously.  All  of  the  mites  are 
sucking  animals,  therefore  the  body  of  a  red  spider  remains  on  the  leaf, 
after  the  attack  of  these  predaceous  species,  or  falls  to  the  ground. 
Predaceous  mites  were  never  common  enough  anywhere  to  become  an 
important  factor  in  the  life  of  the  red  spider.  Certainly  none  of  the 
enemies  of  the  latter,  as  observed  in  southern  California,  could  accomp- 
lish anything  towards  checking  the  increase  of  this  creature.  The 
fact  was  notable  that  the  orchards  first  to  become  freed  from  the  red 
spider  last  spring  were  not  the  ones  in  which  we  found  the  predatory 
forms  most  abundant.  Under  the  conditions  existing  they  constituted 
no  appreciable  factor  in  the  decrease  of  numbers  that  occurred  in  the 
latter  part  of  May,  and  in  June. 

Diseases. — In  the  orchards  where  the  rapid  decrease  of  the  red 
spider  was  first  observed,  the  presence  of  a  fungus  on  dead  mites  was 
noted  and  collections  were  made  in  other  orchards  in  order  to  determine 
its  presence  there.  While  the  fungus  was  noted  almost  everywhere, 
still  it  was  by  no  means  so  prevalent  where  the  mites  had  not  yet  begun 
to  die  so  rapidly.  After  the  mites  had  almost  disappeared  from  an 
orchard  the  remains  of  the  red  spider  could  be  collected  quite  abundantly 
by  beating  the  branches  over  a  glazed  paper  tray,  and  their  mummies 
almost  invariably  showed  the  presence  of  the  fruiting  threads  of  the 
fungus,  extending  out  from  the  body.  The  full  account  of  this  fungus 
will  be  reserved  for  the  final  report  on  the  red  spider.  The  presump- 
tive evidence  that  we  have  here  a  fungous  disease  seemed  quite  strong. 
Death  was  certainly  not  brought  about  by  the  condition  of  the  weather 
directly,  for  in  two  orchards  close  together  one  may  be  badly  infested 
and  the  other  almost  freed  from  the  mite  at  the  same  time.     We  were 


14  UNIVERSITY   OF   CALIFORNIA — EXPERIMENT    STATION. 

not  able  to  produce  the  disease  at  will,  however,  either  in  the  orchard 
or  in  our  breeding  cages;  and  the  fungus  is  not  one  supposed  to  pro- 
duce insect  diseases,  but  belongs  to  a  group  ordinarily  found  on 
decaying  substances.  It  would  seem  that  if  the  death  of  the  red  spiders 
had  been  caused  by  any  infectious  disease,  our  experiments  should 
have  given  more  definite  results.  Further  experiments  with  this  possi- 
ble disease  of  the  red  spider  were  of  necessity  deferred  until  another 
appearance  of  this  mite  gives  opportunity.  The  definite  relation  of  the 
red  spider  to  the  season  of  the  year  indicates  that  if  there  is  a  disease 
that  is  accountable  for  the  diminution  in  the  spring,  it  is  one  very 
dependent  on  weather  conditions. 

Remedies  for  the  Red  Spider. 

The  red  spider  of  the  citrus  trees  is  so  dependent  upon  the  proper 
external  conditions  that  it  would  seem  that  the  most  promising  line  of 
investigation  would  be  to  determine  what  the  cause  of  death  is  at  the 
time  of  their  sudden  decline  in  number,  in  the  hope  that  one  might 
artificially  bring  about  those  conditions.  Much  of  our  work  has  been 
directed  toward  that  object,  but  the  results  have  as  yet  been  chiefly  the 
elimination  of  the  possibility  of  the  natural  enemies  being  any  large 
factor  in  the  problem.  The  approaching  season  of  rapid  increase  may 
throw  a  good  deal  of  light  on  the  subject.  For  the  present  at  least  we 
must  depend  on  the  use  of  insecticides. 

Fumigation. 

The  effectiveness  of  cyanide  fumigation  against  most  kinds  of 
insects  might  lead  one  to  expect  that  it  would  prove  useful  in  com- 
bating the  red  spider.  Hydrocyanic  acid  will  destroy  the  mite  if  used 
in  large  enough  doses  and  for  a  long  enough  time.  It  appears  to  be 
decidedly  more  resistant  than  the  scale  insects,  however.  Now,  for  the 
destruction  of  the  latter,  the  practice  is  to  use  about  as  much  as  the 
tree  will  stand  without  serious  injury  to  the  foliage;  but  this  strength 
is  not  sufficient  to  do  satisfactory  work  in  the  killing  of  the  red  spider. 
It  is  the  uniform  experience  of  those  who  have  observed  the  effects  of 
the  gas  when  fumigating  trees  infested  with  red  spiders,  that  the  latter 
are  not  appreciably  diminished  in  numbers  by  the  treatment.  What 
is  still  more  curious,  fumigated  trees  appear  to  be  more  favorable  to 
the  red  spider  than  are  adjacent  untreated  trees,  and  are  commonly 
more  injured  by  the  mite  during  the  months  immediately  following  the 
fumigation.  The  experience  seems  to  be  extensive  enough  to  warrant 
the  statement  that  fumigation  is  of  no  value  whatever  as  a  means  of 
controlling  the  red  spider. 


RED    SPIDER — REMEDIES.  15 

Sulfuring. 

The  common  red  spider  of  deciduous  trees  has  been  quite  satisfac- 
torily controlled  by  the  persistent  use  of  sulfur.  The  practice  is  to 
dust  the  trees  with  the  dry  powder,  applying  it  at  the  rate  of  about 
twenty-five  pounds  to  the  acre  at  each  application.  The  method  varies 
from  throwing  the  sulfur  into  the  trees  by  the  hand,  to  the  use  of  the 
seed  sower  or  a  rotary  bellows  geared  to  the  wheel  of  a  heavy  farm 
wagon.  The  application  is  repeated  whenever  there  is  evidence  of  the 
beginning  of  a  new  attack,  which  may  be  several  times  in  a  single 
season.  The  action  of  the  chemical  is  supposed  to  be  the  same  as  its 
action  on  mildew,  for  which  it  is  used  in  the  same  way, — that  is,  the 
vapor  produced  by  the  heat  is  fatal  to  them .  The  effectiveness  of  this 
treatment  depends,  therefore,  upon  the  occurrence  of  hot  days.  The 
species  for  which  it  is  used  does  its  worst  work  in  the  hottest 
part  of  the  season.  Where  the  summers  never  get  hot,  as  at 
Berkeley,  the  red  spider  of  the  deciduous  trees  never  does  any  serious 
injury  though  it  is  able  to  live,  and  appears  to  be  always  present. 
The  red  spider  of  the  citrus  trees  requires  some  degree  of  dryness  for 
its  best  development;  this  is  usually  not  great  near  the  coast,  but 
still  it  is  not  able  to  do  much  during  the  hotter  part  of  the  year. 
Sulfur  cannot  therefore  be  expected  to  be  as  reliable  against  this  mite, 
because  of  this  difference  in  season.  We  know  of  no  experiments  on 
a  scale  large  enough  to  fairly  test  the  matter,  but  smaller  experiments 
have  given  entirely  negative  results.  It  is  probable  that  a  few  warm 
days  in  mid-winter  would  make  sulfuring  quite  efficient,  but  as  warm 
weather  cannot  be  depended  upon,  sulfuring  can  at  best  be  con- 
sidered simply  as  a  supplementary  measure,  useful  only  under  favorable 
conditions. 

Spraying. 

As  matters  now  stand  the  only  really  effective  method  of  treating 
the  red  spider  is  the  use  of  an  insecticide  applied  as  a  spray.  There 
are  several  substances  that  will  kill  it  quite  readily,  at  least  in  its 
active  condition,  and  it  is  possible  to  prevent  its  injury  almost 
entirely  by  this  means.  To  do  the  best  work  it  has  been  found 
necessary  to  make  two  or  three  applications,  and  it  is  therefore  quite 
expensive.  In  the  case  of  injury  to  a  tree  produced  in  the  way  this 
creature  works,  it  is  very  difficult  to  make  more  than  a  rough  guess 
as  to  the  amount  of  loss  occasioned,  and  therefore  of  the  amount  it  is 
profitable  to  expend  for  the  control  of  the  pest.  It  is  certain  that 
where  there  is  no  scarcity  of  water  and  the  orchard  receives  the 
proper  attention,   the  mite  is  not  likely  to  materially  decrease  the 


16  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

growth  of  the  tree.  How  completely  the  injury  to  the  crop  can  be 
mitigated  by  giving  the  trees  some  extra  care  remains  to  be  deter- 
mined. Probably  under  most  conditions  the  killing  of  the  spider 
would  result  in  a  saving,  far  more  than  enough  to  pay  for  the 
number  of  sprayings  necessary. 

Citrus  trees  are  extremely  difficult  to  spray  effectively.  To 
do  really  effective  work  every  leaf  of  the  tree,  as  nearly  as  possi- 
ble, should  be  thoroughly  wetted  on  both  sides.  The  web  made  by 
these  mites,  and  their  habit  of  chosing  protected  points  for  their  trans- 
formation and  egg-laying,  requires  that  the  spray  should  be  applied 
with  good  force.  The  way  the  leaves  are  arranged  on  an  orange  tree 
makes  it  almost  impossible  to  spray  the  tree  thoroughly  from  the  out- 
side. The  red  spiders  are  wholly  on  the  foliage,  so  that  one  does  not 
have  the  same  problem  as  when  attempting  to  spray  for  scale  insects; 
but  the  problem  is  none  the  less  difficult. 

The  difference  between  thorough  and  careless  sprajdng  is  very  little 
in  time  or  material  but  amounts  to  a  great  deal  in  efficiency.  One 
should  clearly  appreciate  all  the  difficulties  and  work  intelligently  and 
with  a  well  thought-out  plan. 

Method  of  Spraying. — The  following  plan  of  operation  is  thought 
to  provide  for  quite  thorough  work,  more  thorough  than  any  of  the 
methods  now  in  use.  The  essential  feature  is  the  provision  for  as 
careful  inside  spraying  work  as  is  the  practice  on  the  outside  of  the 
tree.  Many  at  present  recognize  the  importance  of  inside  spraying, 
and  have  adopted  the  practice  of  inserting  the  extension  rod  into  the 
tree  at  several  points  for  this  purpose.  It  is  possible  for  this  method 
to  result  in  a  thorough  treatment  of  the  inside,  but  one  cannot  be  sure. 
It  is  something  like  spraying  the  tree  in  the  dark.  The  practice  here 
proposed  is  to  actually  enter  a  tree  and  see  what  one  is  doing.  It  will 
be  most  convenient  to  spray  the  interior  first,  and  for  this  a  hose  and 
nozzle  without  extension  rod  will  be  necessary.  One  man  may  confine 
his  attention  to  the  interior,  and  the  other  to  the  outside.  The  plan 
to  be  followed  for  inside  spraying  is  shown  in  figure  4.  The  sprayer 
forces  his  way  in  between  the  branches  to  the  center  of  the  tree  and 
begins  to  spray  at,  say,  his  left  side,  spraying  thoroughly  from  the 
ground  up  to  a  line  somewhat  above  the  level  of  his  head.  The  dotted 
line  in  the  figure  indicates  approximately  the  course  of  the  nozzle  as  he 
sprays  the  far  side  of  the  tree  working  around  among  the  branches. 
The  coures  of  the  nozzle  on  the  near  side  of  the  tree  is  indicated  by  the 
solid  lines.  After  the  lower  part  of  the  tree  is  thus  thoroughly  sprayed 
the  upper  part  may  be  treated  as  indicated,  the  operator  of  the  nozzle 


RED    SPIDER — REMEDIES. 


17 


finally  backing  out  while  doing  the  portion  that  stands  directly  over 
his  head. 


Fig.  4. 
plan  for  inside  spraying. 

The  object  of  spraying  the  lower  part  of  the  tree  first  is  that  in  this 
way  the  sprayer  will  not  be  troubled  by  dripping  from  the  leaves  above 
him  and  will  be  more  likely  to  do  careful  work. 

The  outside  procedure  is  represented  in  figure  5.  The  letters  a,  b, 
and  c  indicate  successive  positions  during  the  process.  The  extension 
rod  is  furnished  with  a  nozzle  giving  an  oblique  discharge.  The  first 
attention  is  given  to  the  top,  which  is  the  most  difficult.  The  operator 
moves  about  the  tree  swinging  his  rod  from  d  to  e  or  from  f  to  g 
until  he  is  sure  that  everything  is  thoroughly  wetted.  He  is  then  ready 
to  begin  on  the  lower  part  of  the  tree.  The  rod  is  now  swung  from 
the  position  h  to  i,  back  and  forth,  as  he  goes  around  the  tree  wetting 
everything  in  his  progress. 

The  above  plan  is  not  expected  to  require  any  more  material  or 
time  than  spraying  would  ordinarily  require  if  done  without  a  definite 
plan,  but  should  certainly  be  much  more  likely  to  wet  every  leaf  with 
the  spraying  material. 

Spraying  Materials. — Only  two  substances  have  been  used  for 
spraying  for  the  red  spider.  One  of  these,  distillate  emulsion,  has 
apparently  accomplished  good  results  but  is  still  in  the  experimental 
stage.     We  have  given  considerable  attention  to  the  subject,  but  have 


18 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 


not  yet  arrived  at  results  that  will  enable  us  to  make  definite  recom- 
mendations. There  is  no  question  of  the  killing  power  of  the  sub- 
stance, but  the  effect  on  the  tree  is  the  matter  that  requires  more 
attention.  The  product  is  not  uniform,  but  could  be  if  we  had  any 
definite  idea  of  exactly  what  we  want,  as  the  refining  companies  are 
anxious  to  produce  an  article  that  will  be  satisfactory.  We  have 
obtained  many  samples  of  crude  oils  and  distillates  and  have  done 
considerable  work  in  testing  them,  and  have  obtained  some  important 
results.     These  will  be  reserved  for  another  bulletin. 


Fig.  5. 
plan  for  outside  spraying. 


A  material  that  is  both  safe  and  effective  is  the  sulfid  of  potash 
spray  mixture.  This  mixture  is  usually  made  according  to  the  following 
formula : 

Potash 32  lbs. 

Sulfur,  finely  ground  37  lbs. 

Salt 2  lbs. 

Water 50  gal. 

This  makes  the  stock-solution  which  is  diluted  with  about  a  hun- 
dred times  as  much  water  for  spraying.  The  potash,  sulphur,  and 
salt  may  be  mixed  together  in  a  large  metal  tub  with  a  little  water, 


RED    SPIDER — SUMMARY.  19 

when  chemical  action  will  at  once  set  in  and  the  whole  mass  will  dis- 
solve and  begin  to  boil  very  vigorously .  After  the  boiling  has  ceased, 
the  water  is  added  and  the  stock-solution  is  made.  It  is  very  doubtful 
whether  the  salt  is  of  any  value  in  this  mixture,  but  it  can  at  least  do 
no  harm  as  it  is  in  such  small  quantity. 

This  sulfid  of  potash  is  of  very  little  value,  at  least  at  this  strength, 
as  an  insecticide,  but  is  effective  enough  against  the  active  stages  of 
the  mite.  In  order  to  kill  the  eggs  and  moulting  forms  it  will  be 
necessary  to  make  three  applications,  separated  by  intervals  of  one 
week.     This  will  make  a  very  clean  sweep. 

Summary. 

The  red  spider  of  citrus  trees,  Tetranychus  mytilaspidis,  does  not 
produce  the  spotting  of  the  fruit  usually  credited  to  it,  but  does  cause 
dropping,  and  also  injures  the  leaves.  The  eggs,  which  are  very 
peculiar,  may  be  found  abundantly  on  the  leaves  and  to  some  extent  on 
the  fruit.  They  hatch  after  a  week  or  two,  disclosing  a  six-legged 
mite,  which  after  about  two  days,  one  of  which  is  spent  in  feeding, 
changes  its  skin  and  becomes  eight-legged.  Two  more  moults  occur, 
requiring  five  or  six  days  each  before  the  adult  mite  appears.  It  feeds 
on  the  leaf  by  making  a  slit,  out  of  which  it  sucks  the  contents  of  the 
cells  beneath.  The  male  is  very  active  and  is  found  mostly  on  the 
older  leaves.  The  female  is  much  larger  than  the  male,  and  lays  a 
great  many  eggs. 

The  natural  enemies  most  abundant  are  the  ladybirds,  the  lace- 
wing  flies,  the  coniopteryx  and  a  number  of  species  of  mites.  They 
do  not  accomplish  much  toward  the  checking  of  the  red  spider.  A 
fungus  was  studied  that  may  be  the  direct  cause  of  the  spring  decrease 
of  the  mite,  but  the  weather  is  the  really  important  item. 

Until  we  learn  more  about  the  reasons  of  the  natural  decrease  of 
the  red  spider  we  will  have  to  depend  on  the  use  of  insecticides. 
Fumigation  is  of  no  value  for  this  purpose.  Sulphur  is  not  as  valu- 
able as  it  is  for  the  red  spicier  of  deciduous  trees.  Spraying  is  a 
difficult  operation  on  dense  trees  like  the  orange,  and  is  done  best  by 
spraying  both  from  the  inside  and  the  outside.  Distillates  may  prove 
to  be  the  best  material,  but  until  we  know  better  how  to  avoid  injury 
to  the  tree,  sulfid  of  potash  is  to  be  recommended. 


